Spectral efficiency refers to an information rate (i.e., excluding error correction code), that may be transmitted over a given bandwidth or communication channel. Spectral efficiency is a measure of how efficiently a limited frequency spectrum is utilized by a physical layer protocol (and/or by a media access control or channel access protocol). Spectral efficiency may also be referred to as spectrum efficiency and/or bandwidth efficiency.
Modulation efficiency is measure of a gross bitrate (i.e., including error correction code) of a transmitted signal (e.g., in bits/second), divided by the bandwidth of the signal.
Forward error correction (FEC) may reduce a bit-error rate of a transmitted signal to permit operation at a lower signal-to-noise ratio (SNR). FEC encoding may also reduce spectral efficiency relative to an un-coded modulation efficiency. For example, a FEC code rate ½ reduces spectral efficiency to ½ the modulation efficiency.
An upper bound of attainable modulation efficiency is defined by the Nyquist rate or Hartley's law. An upper bound for spectral efficiency without bit errors in a channel at a given SNR is defined by the Shannon or Shannon-Hartley theorem.
Conventional standards for cable modems specify multiple FEC block sizes, FEC code rates, and quadrature amplitude modulation (QAM) constellations. For a given FEC block size and code rate, each QAM constellation provides an acceptable BER above a SNR threshold.
Conventionally, spectral efficiency versus SNR (SEvSNR) is controllable through selectable FEC code rates. Supporting multiple code rates increases system complexity. In addition, lower code rates reduce efficiency in terms of low-density parity-check (LDPC) encoder/decoder iterations.